A senior Russian naval officer says a torpedo explosion sank the nuclear submarine Kursk last August in the Barents Sea, killing all 118 aboard.

Vice-Admiral Valery Dorogin said a small explosion was followed by a large one in the torpedo area of the submarine.

It is the first time the government commission investigating the tragedy has acknowledged that a torpedo caused the accident - the view widely held in the West.

But Vice-Admiral Dorogin said the cause of the first blast had not been established, leaving open the possibility that the Kursk struck another vessel or a mine on 12 August.

He said there were three possible explanations for the first blast:

an internal malfunction

a collision with a foreign submarine

impact from a mine

He said the third cause was highly unlikely.

Liquid fuel

''It is evident that we will never know what the cause of the first explosion was,'' he told a news conference.

The Russian Government has previously focused on the theory that the Kursk was hit by a foreign ship, and played down the view of international experts that the most likely cause was a torpedo malfunction.

There were no survivors

United States officials say that there were two explosions on board the submarine, the first equivalent to that of a torpedo warhead.

The Kursk was carrying torpedoes powered by a liquid peroxide fuel, a substance tried by Britain's Royal Navy in the 1960s but rejected for being too unstable.

It is believed that a torpedo may have misfired on the Kursk, and exploded in the tube.

Salvage plans

Britain and the US had submarines in the general area of the Russian naval exercise at the time, but insist that they were not near the Kursk.

Earlier this week, the Russian Government said it was finalising plans to raise the Kursk, a project expected to begin in April.

The bodies of 12 sailors were recovered in November before salvage efforts were abandoned because of the dangerous conditions.